[mlir][docs] Add missing parentheses in example code on walkers.

`getFunction` was missing parentheses.

Reviewed By: ftynse, mehdi_amini

Differential Revision: https://reviews.llvm.org/D123999

mlir/docs/Tutorials/UnderstandingTheIRStructure.md

@@ -192,7 +192,7 @@ Operation, for example the following will apply the callback only on `LinalgOp`
 operations nested inside the function:
 
 ```c++
-  getFunction.walk([](LinalgOp linalgOp) {
+  getFunction().walk([](LinalgOp linalgOp) {
     // process LinalgOp `linalgOp`.
   });
 ```